Epoxy resins and epoxy resin compositions essentially containing the epoxy resins as curing agents are widely used in electronic components of, for example, semiconductor devices and multilayer printed circuit boards because cured products thereof can have both excellent heat resistance and insulating properties. In particular, in the technical field of insulating materials for multilayer printed circuit boards in which such resins and compositions are used in electronic components, increases in the transmission speed and frequencies of signals in a variety of electronic devices have been recently promoted. Such increases in the transmission speed and frequencies of signals, however, have caused typical epoxy resin compositions and cured products thereof to have difficulty in terms of required properties of sufficiently low dielectric constant and dielectric dissipation factor. Hence, a thermosetting resin composition which enables production of a cured product having both a sufficiently low dielectric constant and dielectric dissipation factor also in faster transmission of signals with higher frequencies needs to be developed.
In a known technique for a material which enables both a low dielectric constant and a low dielectric dissipation factor, an active ester compound produced through esterification of a dicyclopentadiene phenol resin and α-naphthol with an isophthalic acid chloride is used as a curing agent for an epoxy resin (see Patent Literature 1). An epoxy resin composition in which the active ester compound disclosed in Patent Literature 1 is used enables decreases in both a dielectric constant and a dielectric dissipation factor as compared with epoxy resin compositions in which known phenol novolac active ester resins are used. Such an active ester compound can be, however, dissolved in limited types of organic solvents and therefore requires use of organic solvents with high environmental impact, such as toluene, for producing varnish, a prepreg, and a cured product. Thus, an epoxy resin curing agent which can give a low dielectric constant and a low dielectric dissipation factor and which can be dissolved also in organic solvents with low environmental impact, such as ester solvents and alcohol solvents, needs to be developed.